I know I asked this question a couple of years ago (the last time I built a payment provider for nop).
Has anyone managed to make a direct connection payment provider for Nop commerce which supports 3D Secure? Is there support for it within the NopCommerce Payment provider pattern yet?
nop or any other cart is really irrelevant considering the way 3dSecure works .
once you redirect the user to the payment page which is on the payment service provider side who knows your returnUrl page, if the payment provider uses 3Dsecure and the Issuer bank does too, the payment provider will redirect the user to the issuer ACS server for code verification . once the verification is done, user is redirected back to your returnUrl anyway.
so it's really irrelevant and you usually do not have to do anything outside the classic implementation of a hosted payment plugin for nop.
Hope this helps.